How to Emulate a FULL OUTER JOIN in MySQL?

Emulating MySQL FULL JOIN

In MySQL, the absence of direct support for FULL OUTER JOIN can pose challenges. To overcome this limitation, a common workaround is necessary.

Explanation:

MySQL employs LEFT JOIN and RIGHT JOIN operations, but not FULL JOIN. Therefore, emulating a FULL JOIN requires a combination of these joins.

Approach:

1. LEFT JOIN:

   • Perform a LEFT JOIN between tables t_13 and t_17, matching the common field "value."
   • This operation pairs all rows from t_13 with corresponding rows in t_17, or with NULL values if there are no matches.

2. RIGHT JOIN:

   • Perform a RIGHT JOIN between tables t_13 and t_17, similar to the LEFT JOIN but with opposite table ordering.
   • This step matches all rows from t_17 with corresponding rows in t_13, or with NULL values if there are no matches.

3. UNION ALL:

   • Combine the results from the LEFT JOIN and RIGHT JOIN using the UNION ALL operation.
   • This merges rows that have matches in both tables and preserves duplicate rows.

4. WHERE Clause:

   • Add a WHERE clause to filter out NULL values in the "val13" field.
   • This eliminates rows where both tables returned NULL, indicating no matches.

5. ORDER BY and LIMIT:

   • Sort the combined result by the coalesced values of "val13" and "val17" to obtain a single sorted list.
   • Optionally, apply a LIMIT clause to limit the number of rows displayed.

The above is the detailed content of How to Emulate a FULL OUTER JOIN in MySQL?. For more information, please follow other related articles on the PHP Chinese website!